1. Field of the Invention
The present invention relates to a wet scrubber which captures and scrubs liquid or solid particles contained in an airflow, and also to a paint spray booth comprising the wet scrubber and capable of capturing and scrubbing paint particles contained in a contaminated airflow discharged from the spray booth.
2. Description of the Related Art
Typically, painting of various kinds of mass-produced products such as car bodies and car parts is carried out in a paint spray booth, in which an object to be painted is sprayed with paint utilizing spray painting equipment. Paint that does not stick to the object to be painted floats in the air as paint mist. During the operation of such paint spray booths, it is necessary to supply continuously fresh outside air to, and to remove the paint mist from, the working area by means of a discharge air managing system. These serve the purposes of maintaining a safe and healthy working environment and assuring the highest quality of paint finish. The paint particles contained in this discharge air must be captured before the airflow exits to the atmosphere to avoid environmental pollution.
There are two known methods for separating paint mist from the air exhaust stream: i) a dry method in which the contaminated airflow is made to pass through a dry filter or screen and the paint particles contained therein are adsorbed or trapped by the filter or the like; and ii) a wet method in which the contaminated airflow is put in contact with and mixed with a liquid, such as water, such that the paint particles contained therein are captured and scrubbed by the liquid. Conventionally, in a paint spray facility for painting large products such as cars, the wet method is adopted.
There are various kinds of wet methods for separating paint mist. Typically, the following known methods and means are utilized:
1. A method in which, utilizing gravity difference between the airflow and the liquid such as water, the airflow is made to pass through the bulk liquid to capture paint particles contained in the airflow; PA1 2. A method in which the liquid such as water is made to spill downwardly, and the airflow is made to pass through a liquid film formed thereby, to capture in the film paint particles contained in the airflow; PA1 3. A method in which the liquid such as water is sprayed to create a large population of liquid drops and the contaminated airflow is made to pass through this liquid mist where the liquid drops contact and capture the paint particles to be removed; PA1 4. A method in which the airflow and the liquid such as water are made to pass through a restriction called a venturi. The turbulence of high-velocity air in the venturi causes break-up of the liquid into small drops that intercept and coalesce with the entrained paint particles; and PA1 5. A method in which the liquid such as water is made to flow downwardly on a plate or the like and the airflow is made to blow on the plate, or, the airflow is made to impinge upon a pool of liquid such as water. The paint particles contained in the air stream having greater momentum impact and are trapped on the surface of the liquid.
Typically, a discharge airflow from a paint spray booth consists of an airflow containing a paint mist that includes paint particles of various diameters. The diameters of these paint particles range from several hundred sum to less than 1 .mu.m. In a typical paint mist, there are more small paint particles than large paint particles.
In conventional wet scrubbers used with a paint spray booth of a car assembly plant, an attempt has been made to improve scrubbing efficiency by increasing the frequency and the speed of the impacts of the discharge air stream flowing from the spray section against a capturing water flow. In this connection, U.S. Pat. Nos. 5,074,238, 5,040,482, 4,700,615, 4,664,060, 4,220,078, and the like disclose various proposals. U.S. Pat. No. 5,074,238 discloses a scrubber having a venturi opening through which a discharge airflow and water pass and a curved baffle where air and water mix. U.S. Pat. No. 5,040,482 discloses a scrubber having two troughs, which supply a sheet of water along an inclined surface and a baffle to intermix the water and paint-laden air. U.S. Pat. No. 4,700,615 discloses a scrubber in which several pools are provided hierarchically such that water runs through the pools in sequence, and a discharge airflow is made to pass through the plurality of water curtains that are formed. U.S. Pat. No. 4,664,060 discloses a scrubber in which a lip is provided in the rectangular venturi to increase the intermixing of the air and water, and a baffle plate is disposed below the venturi throat. U.S. Pat. No. 4,220,078 discloses a scrubber with a V-shaped impingement member disposed in the path of a discharge air-paint flow, and a shroud is provided around the collision to effect further scrubbing.
It has been found that attempts to scrub paint particles more efficiently tend to cause increased processing noise. Also, the necessity of increasing the capacity of an exhaust air fan or the like tends to increase equipment cost and energy consumption. Therefore, a device is needed that not only improves efficiency but also reduces noise and energy consumption as much as possible. Reduction of noise is desired from the standpoint of improving the working environment of an operator. U.S. Pat. No. 5,100,442 discloses a scrubber in which a discharge airflow and a water flow are directed into a venturi. Then, they are introduced into a restriction that defines a noise barrier that prevents noise caused by turbulent mixing to pass upstream. U.S. Pat. No. 5,020,470 discloses a scrubber having an elongated discharge tube through which discharge air and water flow. Particulate is removed by virtue of impact of the airflow with an impact pool. Little or no water dispersal or atomization occurs near the top of the discharge tube, and noise is abated. U.S. Pat. No. 4,515,073 discloses a scrubber having a serpentine path in which the air passes through the scrubbing fluid spray several times. A sound absorber is provided within baffles to reduce impact noise. U.S. Pat. No. 4,350,506 discloses a scrubber with a bell-shaped venturi portion that has an enlarged middle and a sound absorber is provided therein. U.S. Pat. No. 4,345,921 discloses a scrubber in which a pair of guide plates is provided in a venturi above the throat to form noise-muffling zones. An impact plate is positioned below the venturi throat and can contain a film or pool of water.
In certain prior-art scrubbers, a portion of the discharge airflow can pass outside the scrubber with little or no mixing with water, and thus can still contain paint particles. Further, the splash of water at a pool can cause contaminated water drops to be discharged with the air via the exhaust air fan. A device to change the direction of the discharge airflow for the purpose of enhancing the scrubbing of paint particles from a paint mist has been proposed in U.S. Pat. No. 4,704,952, for example. This patent discloses a scrubber having structures through which paint-laden air and water flow downwardly and mix together. Partitions outside the structures cause the air to turn abruptly upwardly and then reverse lateral direction. The air passes through baffles and then is discharged into the atmosphere.
Although the prior art discloses many wet scrubbers, there still remains room for improvement. For example, in some conventional wet scrubbers, there are corners and edges, uneven portions, and the like in the path through which air flows and where the air stream is mixed with the water. This results in unnecessary pressure loss, waste of energy, and increased noise. Further, some conventional wet scrubbers still have low efficiency when capturing very small paint particles in water, still present the problem of allowing part of the paint mist to be discharged to the environment, and still permit a large amount of paint-laden water drops to be discharged through the air fan device to the atmosphere.